Telecommunication satellites include antennas that can generate mobile beams for broadcasting multimedia services. These services require the communication networks to be able to cover wide geographic areas and maintain a sufficiently high signal quality over the entire area to be covered. For this, there are mobile-beam antennas that can modify the pointing direction of the beam in order to meet the needs of the telecommunication services.
The telecommunication satellites receive data from the ground stations then they transmit these data to the Earth by means of antennas positioned facing the Earth. Double-reflector passive antennas are preferentially used because they offer the best trade-off between the weight, bulk, efficiency and cost constraints. This is because these double-reflector antennas make it possible, with a given equivalent focal length, to reduce the bulk of the antenna in comparison to a single-reflector antenna. This offers a particularly interesting advantage for reducing the bulk of a satellite in a launch vehicle.
The known passive antenna solutions for displacing a beam of radiofrequency signals over the earth's surface are antennas comprising means that allow for the movement either of the complete antenna mounting or of only the reflector by changing the orientation of the reflecting surface. The existing passive antenna mounting solutions comprise a feed for transmitting and/or receiving RF signals, one or more reflectors and a supporting base to bear all the radiofrequency components of the antenna. There are a number of types of mounting, among which the mountings of Cassegrain type and of Gregorian antenna type can be named by way of indication.
Also known from the prior art is the patent document EP0139482. This document discloses a mobile-beam antenna in which only the reflectors are displaced. It relates to a fixed feed mounting in which the focal point of the primary reflector is maintained on the focus of the secondary reflector.
In the case of movement of the complete antenna mounting, that is to say the assembly consisting of the supporting base, the reflector(s) and the transmission and/or reception feed, the design of the link between the feed and the payload of the satellite becomes problematical. This is because it is necessary to use deformable waveguides or rotating joints which have the following drawbacks: radiofrequency signal losses, frequency band limitation, power limitations, mechanical limitation and numerous actuations and limitation on the numbers of ports at the antenna radiofrequency interface. Furthermore, the deformable waveguides generally have a stiffness that can be significant, resulting in additional stresses on the kinematic means of the antenna mounting. The latter in fact have to be dimensioned so as to be able to deform these mechanical parts.
These many drawbacks linked to the mobility of the feed can be resolved by an alternative antenna mounting solution for which only the orientation of the reflector is modified. However, in the case of a parabolic reflector, when its orientation is modified, the focus of the parabola is also offset from the phase center of the feed. This offset results in a distortion of the radiofrequency geometry leading to degraded efficiency through the focusing aberrations.